The present invention pertains to the field of free-space optical communication, particularly of the type used for two-way satellite communication, i.e. with an incoming Rx signal and an outgoing Tx signal. Effective communication between lasercom terminals requires precision pointing and tracking of the signal beams, and fine beam steering control is significant. In previous systems, rotating deflecting mirrors were the only reliable means of providing the required speed, range, accuracy and repeatability of beam steering.
The use of rotating mirrors in previous systems necessitates a complex optical subsystem, including at least one telescope with an eyepiece to form an afocal reducer, at least one afocal relay (or two relays for a telescope without a field lens) and a focusing lens. Large transverse magnifications are required for an afocal reducer to produce a large field of view in the image space. Also, to achieve high pointing accuracy, the entrance and exit pupils of the system must be well defined, and they must coincide with the respective steering mirrors. Thus, multi-element lens groups must be employed for relays, eyepieces and detector optics. Such systems are costly to manufacture and also have high mass which is undesirable for deployment and operation in satellite applications.
In addition to the above, the optical subsystem for the previous lasercom terminal design requires a large number of optical surfaces. Each optical surface contributes deformation to the optical wavefront due to optical material imperfections, manufacturing errors, assembly and alignment errors and environmentally-introduced errors, such as temperature gradients and vibrations. However, satellite lasercom systems must propagate efficiently over thousands of kilometers, and must provide efficient coupling to the internal detector components. Thus, wavefront error must be minimized. A state-of-the-art optical terminal must be diffraction-limited with a very small wavefront deformation budget (i.e. total cumulative RMS error smaller than 0.07 of a wavelength of laser light). It is a technically challenging task to meet such a requirement in a system involving so many optical surfaces.
Further, each optical surface inherently scatters a portion of the beam, further reducing efficiency and elevating the stray light level. This can result in optical crosstalk between the transmission and receiving channels, in addition to other errors such as lower than expected BER, burst errors, and failure to acquire and track a signal in the presence of background radiation (such as from celestial sources, i.e. the Sun, Earth, Moon and planets, and also reflections and scatter from the satellite). Typical previous optical architectures require 50 optical surfaces or more, resulting in significant contributions to the above-noted problems.
There are other sources of optical crosstalk, and several design strategies are typically implemented, such as carefully designing baffles and stops, and using narrowband spectral filters. In addition, there are several known approaches at the optical subsystem level for reducing crosstalk. FIG. 1 shows a previous system that provides complete spatial separation of the Rx and Tx signals by using two telescopes 10, 12. While this design offers excellent control of stray light, it creates other problems such as significantly increasing mass, difficulty in alignment increased optomechanical complexity, and numerous optical surfaces in the subsystem.
Another approach is shown in FIG. 2, in which partial spatial separation is accomplished with a single telescope 20 and a shearing aperture 22, where Rx and Tx signals share the same optical path. The Tx source 24 is drawn off a separate optical path from the Rx detector 26 by e.g. a reflecting mirror 28. While lighter in weight as less complex than the two-telescope design, the shearing aperture approach has poor stray light control since it is impossible to isolate the Tx beam from the Rx beam.
FIG. 3 illustrates polarization isolation, in which a single telescope 30 is used, and the Rx and Tx signals are distinguished as perpendicular polarized beams separated by a polarizing diplexer 32, which sorts the Rx and Tx beams along separate optical paths. This method requires the highest quality polarization components and extremely tight control of the state of polarization as the Rx and Tx beams propagate through the subsystem.
FIG. 4 illustrates dichroic isolation, in which a single telescope 40 is used, and different laser wavelengths are used for the Rx and Tx channels. The separate optical paths are defined by a dichroic beamsplitter 42. As a result, the two communicating terminals are precluded from operating at identical wavelengths.
In previous systems, as shown in FIG. 5, track detection is accomplished using a fast steering mirror (FSM) 50 which shifts in position to correct for mispoint in Rx channel beam. A beam splitter 52 directs the Rx signal to an Rx detector 54, and directs a portion of the Rx signal to a track detector 56. Since the Rx detector 54 and the track detector 56 are aligned, the FSM 50 cancels out mispoint on both simultaneously. However, the physical size of the track detector 56 limits the field of view of track detection. Thus, previous tracking systems are only sensitive to small angular mispoints, which makes signal acquisition and tracking more difficult.
In all previous designs, boresight alignment also poses a major technical challenge. Typically, for communication distances greater than 4000 km, the communication Rx and Tx beams should be aligned within 1-3 microradians in free space. The corresponding misalignment requirement between channels may be even less than one microradian. While such boresight alignment is difficult to achieve during manufacture, it is even more difficult to maintain during launch deployment and on-orbit operation. Due to the long optics required for previous systems, the channel inputs must be separated by a great physical distance, which requires an extremely stable and expensive mechanical structure with tight controls over environmental variables.